Docker Compose

Compose describes a whole application, its services, networks and volumes, in one compose.yaml file. docker compose up creates and starts everything, in the right order, with one command.

How it works

Each service is a container definition: an image or a build:, ports, environment, volumes, restart, healthcheck. Compose puts every service on the project's network and recreates only the containers whose configuration changed. docker compose down removes containers and networks but keeps named volumes, unless you add -v.

What breaks

YAML is strict: indentation decides the structure, and port mappings like "22:22" must be quoted or YAML may read them as a number. docker compose config prints the file as Compose understands it, after variables and overrides.

In compose.yaml

services:
  api:
    build: ./api
    environment:
      DATABASE_HOST: db
    restart: unless-stopped
  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: ${DB_PASSWORD}
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

See it break, then fix it

Write the Dockerfile or the compose file, type the docker commands, and watch a simulated server run them through reboots, upgrades and traffic. Free to start.

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